1. Field of the Invention
This invention relates to a method and apparatus for accurately estimating the concentration of residual fumigant in a commodity in a fumigation chamber from the decline in concentration of fumigant in the exhaust air stream during the aeration period following fumigation.
2. Description of the Art
Gaseous fumigants are important for the commercial control of pests in a variety of commodities. Methyl bromide (MB) is a highly toxic, colorless, odorless gas that is widely used as a fumigant for fresh fruit and other agricultural products. This highly penetrating gas can kill insects and other arthropod pests within the commodities. When the commodities are treated with MB, a small portion of the gas reacts with the product producing relatively innocuous inorganic bromide residue, and the remaining MB desorbs from the product. Free MB, however, can persist in commodities for some time after fumigation, and exposure through inhalation and ingestion may be hazardous to workers that handle fumigated commodities and to the consumer. MB residues have become a concern of the United States Environmental Protection Agency (EPA), and residue data for a large number of agricultural products is required to support continued use of this important fumigant.
In the Pacific Northwest a treatment using MB as a fumigant is being developed for apples to satisfy import quarantine requirements for Japan and other countries. Fumigation would be followed by an aeration period in which fresh air is forced through the fumigation chamber to remove gaseous MB from the free air space. Animal Plant Health Inspection Service (APHIS) guidelines require that aeration must continue until the concentration of MB in the exhaust air stream evacuated from the chamber is 5 parts per million (ppm) or less. This requirement is intended to protect workers who enter the chamber immediately after the aeration period and is not concerned with residues of MB in the commodity. As a result of this requirement, the length of the aeration period is determined by the ventilation rate, because higher ventilation rates more effectively dilute the MB in the exhaust air stream and as a result shorten the aeration periods. Present regulations do not specify a tolerance for free MB in apples, although a tolerance of 0.2 ppm has been proposed (EPA, Federal Register 51: 9512 (1986)). Previous work on residues of free MB in sweet cherries has shown that the aeration periods commonly used in commercial fumigations are usually too short to reduce MB to below 1 ppm (Sell et al., Pesticide Science 23: 41-49 (1988)). It is highly desirable to remove MB residues from commodities during the aeration period, before there is human contact with the treated commodity. Presently, the way to precisely determine MB residue concentrations in fumigated commodities is the headspace analysis method. This is a destruction method wherein a sample of the commodity is homogenized in water, and the headspace (gaseous phase above the homogenate) is assayed using gas chromatography. (See King et al., Journal of Agricultural and Food Chemistry 29: 1003-1005 (1981)). While useful for laboratory studies where experiments can be carried out in a fume hood, headspace analysis cannot be used to determine residue concentration during aeration in a commercial fumigation chamber because the chamber would have to be entered to remove the sample for analysis. This would be highly hazardous to workers.
Another commercially important fumigant for control of pests in a variety of commodities is ethylene dibromide (1,2-dibromoethane, EDB). Although EDB has been removed from the U.S. market, it is still used as a fumigant in other markets in the world. As with MB, precise determination of EDB residues requires destructive testing that can result in exposure of the workers to the gas. What is needed is a method for determining fumigant residue concentrations in treated commodities which is nondestructive, minimizes exposure of workers to the fumigant, and is suitable for commercial use.